Our efforts in the coming year will be aimed at a more detailed analysis of the ontogeny and developmental potential of TdT positive cells. One of the major thrusts will be to use the fluorescence-activated cell sorter to isolate highly purified populations of TdT positive cells for use in in vivo reconstitution studies. We hope to demonstrate directly that many of these cells are thymocyte progenitors, and to test their potential for generating B cells as well. As part of this study, we will attempt to induce T cell differentiation in a culture system which selectively promotes the proliferation of TdT positive cells in vitro. Another area of investigation will concern the fate and developmental potential of a population of TdT positive cells that is present in the blood of young animals. Lastly, we plan to investigate the cellular origins and sites of neoplastic transformation of MuLV-induced thymic lymphomas in rats.